1. Field of the Invention
The present invention relates to a process for preparing an optical element to be placed in a tubular jacket to form optical transmission means such as a tele-cable, and which comprises an optical fiber of glass or resin, which possibly is provided with a tight fitting coating and which is located in a plane or spatial curve with a radius of curvature being sufficiently big to avoid a substantial increase of the loss at the light transmission. The invention further relates to optical elements so produced and to optical transmission means comprising such optical elements.
2. Description of the Prior Art
It is known to produce tele-cables with optical fibers of glass or a plastic material in which the fibers are placed in the cables in longitudinal spaces with considerably larger cross-sectional dimensions than the fiber diameter, cf. for example the Swedish patent application No. 75,08599-3, (DT-OS No. 25 28 991). It is also known to establish a twisted arrangement of the fibers in such spaces. It is thus especially known from DT-AS No. 24 25 532 to place an optical fiber wave-shaped preferably in one plane in a tubular protective jacket. As far as possible the wave shape is determined by means to ensure a securing of the optical guide which entails a considerable risk in respect of formation of micro-bendings at the securing members. The wave shape is established purely mechanically by a reciprocating movement before the placing in the jacket, which furthermore in practice involves serious difficulties of adaptation. Further, it is known to adhere fibers in undulated paths between two plastic bands which then are wound round a massive support wire, cf. U.S. Pat. No. 3,937,559 (DT-OS No. 24 24 041). Moreover, cables are known where the fibers are helically wound around a soft support layer placed around a central reinforcing member cf. U.S. Pat. No. 3,883,218 (DT-OS 23 55 854). It is a common aim of these known cable types to prevent as far as possible the mechanical impacts on cables at tension or bending from affecting the optical fibers, and especially to prevent the occurence of detrimental tensile impacts.
DT-OS No. 23 02 662 describes a tele-cable containing several optical fibers where several loosely wound separate fibers provided with a protective layer are gathered in basal bundles being provided with a common protective coating. Several basal bundles are gathered to a cable, and the spaces between the basal bundles as well as between the individual fibers are filled with a sliding medium.
This structure with loosely wound fibers entails during maneuvering and handling of the cables a considerable risk of cumulative displacements of the fibers with a consequent risk of vigorous local forces which can reduce or destroy completely the transmission properties of the fibers.
DT-OS No. 24 30 751 describes a tele-cable having a structure of several layers of glass fibers surrounded by a protective sheath of elastomer being wound helically in layers where the protective sheaths are interconnected, and where the pitch is selected in consideration of the choice of material so that the cable has a controlled extension over a wide temperature range. This connection between the protective sheaths means in practice that the cable behaves as if the fibers were cast-in.
This cable is extremely difficult to construct because in practice it has to be built-up over a cylindrical body, requires special tools and makes very specific demands in respect of material properties and angle of pitch.
The U.S. Pat. No. 3,328,143 describes the preparation of multi-fiber structures where the separate fibers are twisted to ensure an intimate contact between these during fusion. Such twisting does not, however, affect the finished fiber bundle.
From DT-OS No. 25 12 006 and also from a number of the above publications it is known to place in a cable an optical fiber with forced helical shape and provided with a sheath. However, all these publications have one thing in common, viz, that there is no preshaping of the element, and DT-OS No. 25 12 006 comprises in particular the feature of a controlled twisting which keeps the fiber in shape and which in a usual manner within the cable technology utilizes steam barrier layer, buffer layer, etc. Thus, the fibers have no free location in a tubular jacket, of the contrary, they are arranged systematically around a central tension relief element.
Finally, from DT-OS No. 25 13 723 flat cables are known in which an optical fiber is placed parallel and in direct connection with a strength member of massive spring steel in a common flat protective sheath. The strength member is rigidly connected with the sheath while the fiber essentially free of tension and without being given a preshaping is located in a space being parallel with the tension relief element in the sheath, but without being rigidly connected with the sheath and without being provided with a tight-fitting coating. Said flat cable is made to be attached directly to the wall or can be shaped especially flexible by making it helically shaped by winding it around a drift so that it is shaped as a helical spring due to the tension element being of spring steel.
There is nothing in the publication, indicating that the flat cable thus formed can, not to say, shall be placed in a tubular jacket. The object of the present invention is to provide constructional characteristics whereby it is possible to a greater extent than before to avoid deterioration or destruction of the optical fibers in respect of their transmission capability as a consequence of mechanical impacts such as tension, bending, torsion and vibration. In this connection it must be remembered that deterioration or even destruction of the transmission capability for an otherwise perfect optical fiber can be expected if due to one or several defects its light conductive interior is narrowed or bent even at an extremely small part of a cable section where the magnitude of the extent of the defect only amounts to a fraction of a millimeter, for which reason such defects are referred to as microcracks or microbendings. It is obvious that the tensile force affecting the optical fiber will increase the possibility of microcracks the bigger the tensile force is. In the heretofore known constructions of tele-cables with optical fibers efforts have been made, as is previously stated, to reduce the magnitude and the risk of tensile stresses in the optical fibers, for example by the said forms of twisted or helical placing of the fibers in the jacket.